1. Field of the Invention
The present invention relates to the manufacturing of high purity optical fluoride crystals, the making of purified optical fluoride crystal feedstocks and to the anionic purification of optical fluoride crystalline materials. The invention relates generally to methods for removing oxide impurities from optical fluoride crystal feedstocks. More specifically, the invention relates to a method for preparing purified optical fluoride crystal feedstocks and the use of the feedstock in manufacturing VUV less than 200 nm transmission optical fluoride crystals for VUV lithography/laser systems.
2. Background Art
Crystals of alkaline earth and alkali metal fluoride salts are useful materials because of their low-wavelength absorption edges. Crystals of fluoride salts such as CaF2, BaF2, SrF2, LiF, MgF2, and NaF are useful in applications that require high transmission in the vacuum ultraviolet (VUV) region, i.e., at wavelengths below 200 nm.
Fluoride crystals are commonly grown using the Bridgman-Stockbarger process, with fluoride crystal feedstocks loaded into a crucible which is disposed inside a hot zone of a vertical furnace. The hot zone is then heated to a temperature sufficient to melt the fluoride crystal feedstock raw material. After melting the fluoride crystal feedstock raw material, the crucible is slowly lowered from the hot zone to a cold zone. As the crucible passes from the hot zone to the cold zone the molten material goes through a zone of thermal gradient. On passing through this zone, the temperature transition inside the molten material creates a crystal front. The crystal front propagates inside the crucible, within the material, as long as the crucible is caused to move downwardly.
The starting materials (fluorides of alkali metals or of alkaline-earth metals) are found on the market with levels of cationic contamination of the order of a ppm. As regards the anionic contamination, these levels are generally higher: of the order of or higher than 100 ppm. Said anionic contamination of this type of material (fluorides of alkali metals or of alkaline-earth metals) is essentially due to oxygenated species. It is highly disadvantageous. CaF2 and BaF2, when they contain oxygen, present a very low transmission in the ultraviolet, with the oxide impurities in fluoride crystals having a degrading effect on VUV transmission of the crystals. The oxide impurities are attributed primarily to the reaction of water molecules with the fluorides and residual carbonates. Unfortunately, it is difficult to avoid oxide contamination in the crystals because water is ubiquitous. Water molecules are usually found in the raw material used in preparing the crystals as well as during the crystal growth process. A common strategy for reducing the oxide content in crystals is to react an oxide scavenger with the raw material prior to growing the crystal, i.e., prior to moving the melted raw material through a thermal gradient. This reaction may be carried out separately from the crystal growth process or as part of the crystal growth process. In a classical processes of preparation of monocrystals according to the Stockbarger technique solid compounds are incorporated such as PbF2, CdF2 or ZnF2, for neutralising the oxygenated species present, which originate from the starting material incorporated and/or from the reaction environment. PbF2 is the compound, called the fluorinating agent, which is the most used to this day insofar as its manipulation does not give rise to any particular difficulty, and insofar as it is solid at ambient temperature and insofar as it has, itself and its corresponding oxide (PbO), a high vapour pressure at the temperatures of use of the crystallisation furnaces. Said PbF2 reacts, within the context of the preparation of crystals of CaF2, notably according to the following reaction: CaO+PbF2xe2x86x92CaF2+PbO.
However, the incorporation of PbF2 and of its homologues, if it is beneficial with regard to the damage from the oxygenated species, it is furthermore disadvantageous. In fact, traces always remain of said solid compounds in the crystal and this consequently affects the transmission of said crystal and the homogeneity of its refractive index at below 200 nm.
The present invention provides a process of anionically purifying a feedstock powder of an optical fluoride crystal of an alkali metal or of an alkaline-earth metal; and a method of preparing an optical fluoride crystal of an alkali metal or of an alkaline-earth metal which includes, in its implementation, said process of feedstock anionic purification.
In one aspect, the invention relates to a method for making a below 200-nm wavelength transmitting optical fluoride crystal feedstock by anionically purifying a powder and the making of a below 200-nm wavelength transmitting optical fluoride crystal from the anionically purified feedstock.
In another aspect, the invention relates to a method for manufacturing an optical crystal for transmitting light of a wavelength less than 200 nm which comprises loading the anionically purified feedstock into a crucible, melting the anionically purified fluoride feedstock, and growing the crystal by moving the melted fluoride raw material through a thermal gradient.
In another aspect, the invention relates to a method for manufacturing an optical fluoride crystal for transmitting light of a wavelength less than 200 nm which comprises loading an anionically purified fluoride feedstock having a maximum oxygen content of less than 50 ppm of oxygen, less than 0.05 ppm of lead, and less than 5 ppm of other contaminants, such as the cationic contaminants, melting the anionically purified fluoride feedstock material having a maximum oxygen content of 50 ppm, and crystallizing the melted fluoride raw material to form a crystal having an internal transmission of at least 99%/cm at 157 nm.
In accordance with the present invention, an optimisation is in fact proposed of the mode of incorporation of reactive gas intended for purifying the feedstock material and the environment of their anionic impurities (oxygenated species).
Other features and advantages of the invention will be apparent from the following description and the appended claims.